Doubt in the code

Projects Programming
2 
enum { Off = HIGH, On = LOW };

#define Interval 3000L

const  byte  pinBut = A1;
const  byte  pinLed = 10;

unsigned long msecLst = 0;
unsigned long msec;

// -----------------------------------------------------------------------------
void
setOutput ()
{
    digitalWrite (pinLed, On);

    msecLst = millis ();
}

// -----------------------------------------------------------------------------
void setup()
{
    Serial.begin (9600);
    digitalWrite (pinLed, Off);
    pinMode      (pinLed, OUTPUT);

    pinMode      (pinBut, INPUT_PULLUP);
}

// -----------------------------------------------------------------------------
void loop()
{
    msec = millis ();

    // check for button change and press
    static byte butLst = Off;
    byte but = digitalRead (pinBut);
    if (butLst != but)  {
        butLst = but;
        if (On == but)
            setOutput ();
    }

    // turn off LED after Interval
    if ( (0 < msecLst) && ((msec - msecLst) > Interval))  {
        msecLst = 0;
        digitalWrite (pinLed, Off);
    }
}
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